This invention relates to a unitary superconducting electromagnet and method of forming same, and more particularly to such a unitary superconducting electromagnet and method which utilizes superconducting coils and an outer ferromagnetic body about the coils in obtaining a desired shape and intensity of the magnetic field.
Heretofore, it has been recognized that magnetic iron saturates at a magnetic field intensity of around one and one-half (1-1/2) Tesla and this has tended to restrict the operating range of conventional electromagnets. While higher magnetic fields have been utilized in conventional magnets, it has been very difficult to obtain a precision magnetic field having a uniformity or homogeneity of at least one (1) part per million. Also, such precision magnetic fields require substantial electrical power thereby resulting in high operating costs.
A superconducting magnet which utilizes superconducting coils substantially decreases the power requirements and permits much larger current densities thus reducing the amount of conductor material required for a specific predetermined current. Some superconducting magnets have used magnetic iron for a shield for the magnetic field but these shielded magnets have been limited to non-precision magnetic fields having a magnetic field intensity not substantially greater than the magnetic saturation of the iron which is around 1.5 Tesla.
Many present uses of superconducting electromagnets require a very high degree of magnetic field uniformity or homogeneity such as at least around one (1) part per million to around one (1) part per one hundred million. Such a uniformity can be obtained only with a magnetic field having a precise shape and intensity. Heretofore, only current has been used to obtain such a magnetic field uniformity.
Magnetic iron shields have been used heretofore for magnetic fields having an intensity not substantially greater than the magnetic saturation of iron, such as in U.S. Pat. No. 4,646,045, dated Feb. 24, 1987 which shows nuclear magnetic resonance (NMR) magnets in medical apparatus in order to minimize or eliminate any magnetic fringe or stray field which could interfere with adjacent equipment operation. A separate shield was utilized in U.S. Pat. No. 4,646,045 with a minimum amount of ferromagnetic material in order to minimize the effect of the shield on the field homogeneity and there was no suggestion of having such a shield effective at magnetic intensities greater than the magnetic saturation of the magnetic iron in the shield, and particularly to increase the magnetic field intensity. At higher magnetic fields above the saturation of magnetic iron, it has been common to provide a magnetic iron shield at a substantial distance from the coils, such as in the walls of a room containing the magnet.
A magnetic iron shield provides a part of the closed magnetic path and affects the shape and intensity of the magnetic field. Thus, it is necessary that the superconducting coils and shield be accurately positioned in relation to each other and to the magnetic field so that the desired shape and intensity of the magnetic field is obtained. Heretofore, such a superconducting electromagnet has not been provided in which the positioning of the coils and ferromagnetic shield in relation to each other and in relation to the magnetic field could be accurately predetermined so that a magnetic field uniformity of at least around one part per million is obtained, particularly when utilized in a magnetic field having an intensity substantially greater than the magnetic saturation of the iron.